1. Field of the Invention
The present invention relates generally to Ground-Fault Circuit-Interrupter (GFCI) systems, and more particularly to a new and improved GFCI system for alternating current, separately derived, three-phase electrical power systems wherein means are provided for continuously monitoring the current balance conditions on the main power supply bus supply lines and on each feeder circuit connected thereto, and in the event that a fault condition is determined to exist based on certain relationships between the sensed currents in the main bus supply lines and the sensed currents in any feeder circuit, then the faulted circuit will be tripped and the other circuits will be inhibited from tripping.
2. Discussion of the Prior Art
Prior art ground-fault protective systems are intended to sense small differences in current in power conductors that normally carry balanced currents. Such differences may be caused by leakages of current from one of the line conductors to ground, thus depriving the neutral conductor of some of the normal current that would establish a balance, or zero difference, in current in the conductors at a sensor. If the differential currents are below certain predetermined levels, power is normally allowed to flow uninterrupted. However, if differential currents should occur that exceed a predetermined threshold for a long enough time, the circuit is interrupted, since it is then probable that an incipient failure of insulation or perhaps even a serious shock to a human being is occurring.
Spurious signals often cause ground-fault interrupters to be confused with real fault currents. For example, power line transients caused by sudden load changes, or lightning induced surges, can give rise to unnecessary line tripping in ground-fault interrupter systems. Since such disconnections of the circuits interfere with efficient system operation, it is not unusual to find that intolerance thereto has caused the users of such equipment to establish sensitivity specifications at dangerously high levels. A steady-state spurious signal frequently experienced in three-phase electrical power systems is a capacitive current to ground from at least one of several downstream feeder lines. This can be caused by a long cable to a load, or by discrete phase-to-ground connected capacitors such as those used to avoid damage to load-utilization equipment by power system voltage surges, or by similar circuit influences having nothing to do with a true fault on the line. It can thus be said that interruptions of the circuit brought about by a ground-fault detector and interrupter system for causes that prove to be insufficient, yet cause the system to respond by needlessly breaking the circuit without the occurrence of a true fault, are a nuisance and must be avoided. A true ground-fault can have different causes and can give rise to different levels of current imbalance in the supply conductors. If the current imbalance is comparatively high; that is to say, if a comparatively large ground-fault current flows, the system should respond quickly and decisively.
Modern GFCI technology has limited application for systems operating above 125 volts line-to-ground or 250 volts line-to-line. Conventional GFCI applications are principally applied to single-phase, 120-240 volt power systems. When the system is a three-phase, multiple feeder circuit system operating above 125 volts-to-ground (e.g., systems rated 400 or 480 volts phase-to-phase, which have a normal voltage-to-ground of 230 and 277 volts, respectively), and one phase is faulted to ground, the magnitude of the capacitive charging currents on the unfaulted phases of the non-affected feeders can easily reach a magnitude that will “false trip” the non-affected feeders' GFCIs. This is not a common problem on systems rated below 125 volts to ground (e.g., a 240-120 volt single-phase system or a 208Y/120 volt three-phase system), because it takes an exceptionally long feeder circuit (with a circuit conductor length of approximately 1000 feet) to result in a capacitive charging current above the GFCI trip level of 4 to 6 mA.
A common voltage used for lighting circuits in the United States is 277 volts phase-to-ground (or phase-to-neutral), which is the voltage to ground or neutral that exists for all three-phase electrical systems rated 480 volts phase-to-phase (except unusual “corner grounded” systems). In a typical situation involving possible electrocution of an individual completing a ground-fault circuit through his body, death does not occur instantaneously, but results most often from ventricular fibrillation. The higher the electrocuting current, the shorter the time in which ventricular fibrillation occurs. Using the 95th percentile human body resistance at 1000 volts (reference IEC TS 60479-1, Fourth Edition, July 2005) yields a “dry” hand-to-hand resistance of 1050 ohms and a dry hand-to-foot resistance of 945 ohms. As an example, the lowest resistance, a dry hand-to-foot resistance of 945 ohms, can be used in a sample calculation for a 690 volt system. At a lower voltage of 225 volts, the dry hand-to-hand resistance is 1900 ohms and the dry hand-to-foot resistance is approximately 1710 ohms. Using these resistances, a hand-to-hand resistance is 1900 ohms corresponds to a body current flow of 146 milliamperes (mA) at a voltage of 277 volts. The hand-to-foot resistance of 1710 ohms corresponds to a body current flow of 162 mA at a voltage of 277 volts. Either of these illustrated levels of current flow are significantly above the threshold of 6 mA where a person can voluntarily “let go” of, or release, a grasped energized conductor. In fact, these magnitudes of current can result in ventricular fibrillation of the heart if the current flow persists through the body for more than approximately one second. In fact, many of the electrocution deaths experienced today are at the 277 volt level.
Ventricular fibrillation is thus considered to be the main mechanism of death in fatal electrical accidents. Ventricular fibrillation results from shock currents through the heart in excess of approximately 40 mA. A published (IEC TS 60479-1, Fourth Edition, July 2005, FIG. 20) time-current plot for various time duration exposures of current flow though the body (for current flow ranging from approximately 40 mA to 1500 mA), depicts a set of probability curves (ranging from a “threshold risk” up to 50% probability) for experiencing ventricular fibrillation. As suggested above, the duration of the shock is a key factor. According to IEC TS 60479-1, “For shock durations below 0.1 s, fibrillation may occur for current magnitudes above 500 mA, and is likely to occur for current magnitudes in the order of several amperes only if the shock falls within the vulnerable period. For shocks of such intensities and durations longer than one cardiac cycle, reversible cardiac arrest may be caused.” Additionally, “The vulnerable period occurs during the first part of the T-wave in the electrocardiogram, which is approximately 10% of the cardiac cycle . . . .” A shock will not necessarily result in an electrocution for body currents of up to several amperes if the voltage source is removed quickly enough. The faster the voltage source is removed from a person, the less likely ventricular fibrillation will occur. Ventricular fibrillation often leads to death unless prompt medical intervention is initiated (i.e., CPR, followed by defibrillation)
The International Electrotechnical Commission (IEC) “c1” empirical curve for the threshold 5% probability of ventricular fibrillation for a left-hand-to-foot shock (heart current factor of 1.0) can be expressed by the equation:t(I)=0.2[(500−I)/(I−40)]0.5
where:                t=time in seconds, and        I=current in milliamperes (mA)        
Calculations pursuant to this equation indicate that a GFCI device must clear 400 mA of current within 0.1 second to avoid ventricular fibrillation for the “worst case” of a shock from the left hand to a foot.
For a 690 volt three-phase system (maximum voltage of 720 volts phase-to-phase):
                              I          body                =                ⁢                              (                          720              /              1.732                        )                    /          945                                                  =                    ⁢                      0.440            ⁢                                                  ⁢            A                          ,                  or          ∼                      440            ⁢                                                  ⁢            mA                              
One fact that has inhibited the application of GFCIs on voltages greater than 125 volts line-to-ground, or on three-phase systems, is that, as pointed out above, all feeder circuit conductors on such power systems have a characteristic capacitance-to-ground. This is referred to as “system charging current” and is described below. The normal system charging current present on all such systems can often exceed the nominal 6 mA threshold of GFCI devices and result in the nuisance tripping of GFCI protected circuits that are not actually involved in the circuit that has a ground-fault.
Referring now to FIG. 1 of the drawing, a three-phase source S is shown coupled via main phase lines A, B, C to a pair of loads LOAD1 and LOAD2 through feeder lines A′, B′, C′ and A″, B″ C″, respectively. This circuit represents a Prior Art GFCI application in which separate multiple GFCI units, such as the depicted units GFSI1 and GFSI2, are used as protective mechanisms in the respective feeder circuits. Shown in dashed lines are capacitive symbols “C0” representing the distributed capacitances-to-ground for each feeder line. The system charging current “IC” for the feeder circuit to LOAD1 can be calculated from the per-phase capacitance-to-ground values using the following equations:IC=3ICO=√3VLL/XcoXco=(106)/2πfCo                where        IC=System charging current during a ground-fault, in amperes;        ICO=System charging current of each phase during normal system conditions (no ground-fault), in amperes [ICO];        VLL=System line-to-line voltage, in volts;        XCO=Per-phase capacitive reactance, in ohms [XCO];        f=Frequency, in Hertz; and        CO=Per-phase capacitance-to-ground, in microfarads.        
Using the above equations for a 13 mA system charging current (IC) at 480 volts (typical for a three-conductor insulated cable circuit in metallic conduit of a 1000 ft length) yields:
                              X                      c            ⁢                                                  ⁢            o                          =                ⁢                  1.732          ⁢                                    (              480              )                        /            0.013                                                  =                ⁢                  64          ⁢                      ,                    ⁢          000          ⁢                                          ⁢          ohms          ⁢                                          ⁢          per          ⁢                                          ⁢          phase          ⁢                                          ⁢          for          ⁢                                          ⁢          a          ⁢                                          ⁢          1000          ⁢                                          ⁢          ft          ⁢                                          ⁢          long          ⁢                                          ⁢          feeder          ⁢                                          ⁢          cable                    
From the prior calculation of “body resistance,” it will be apparent that when a person touches an energized electrical phase conductor, it is equivalent to putting a resistor in the order of 1050 ohms in parallel with a −j64,000 ohm capacitive reactance XCO, except that the capacitance is distributed along the entire cable leading to the source, and most of the current will take the more direct path through the body resistance. (Note: RN in FIG. 1 is the system's neutral grounding resistor and can vary from zero resistance for a solidly-grounded system, to a few hundred ohms for a high-resistance grounded system, to an infinite value for an ungrounded system.
In the illustrated example, a fault in any of the feeder lines to LOAD1 will be sensed by GFCI1. Note that as depicted, GFCI1 includes a circuit breaker CB1 and a ground-fault sensor detection device GFS1 that is coupled to an overall core-balance, current transformer CT1 that encircles all three phases A′, B′ and C′ (as well as the neutral for a three-phase, four-wire system if used). Each of the capacitive charging currents in the three-phase load conductors (and neutral) sum to zero for a balanced or unbalanced load condition. Under normal system operating conditions, the capacitive charging currents ICO in all three phases are equal and sum to zero.
In this example, the fault current induced on the multi-turn secondary winding W1 of CT1 is proportional to the vectorial sum of the capacitive charging currents flowing in the three line conductors A′, B′, C′. As long as this sum is below a predetermined threshold value (typically 4 to 6 mA), the net flux induced in the core of CT1 and correspondingly, the fault current induced on its multi-turn secondary winding W1 and coupled into GFS1 will be beneath the trip threshold thereof.
In the absence of an induced fault current in winding W1 exceeding the threshold level, the differential current transformer remains correspondingly “balanced”, and circuit breaker CB1 is held in its closed state. However, should a fault to ground occur, such as is shown at “F” in FIG. 1, where line A′ is shorted to ground, the vectorial sum of the capacitive charging currents in lines A′, B′, C′ will no longer be less than the threshold value, and the corresponding fault current induced in the secondary winding W1 will cause the differential transformer of GFS1 to become unbalanced, and trip circuit breaker CB1 to interrupt the feeder circuit to LOAD1 and clear the ground-fault F.
But in addition, as may be further noted in FIG. 1, and as will be further discussed below, during the fault, the unbalanced voltages that exist with respect to ground also force current flow (currents Ib2 and Ic2) in phases B″ and C″ of the feeder circuit to LOAD2 (and any other feeder circuits in the system driven by source S). These two currents can result in a false trip of the non-faulted feeder circuit if the resulting unbalance causes the generation of a fault current in W2 that exceeds the trip threshold of GFS2. This of course causes an unnecessary “nuisance” trip and should be avoided.
There is thus a need for a GFCI system for three-phase applications principally operating at voltages above 125 volts and having a ground-fault pickup sensitivity of 4 to 6 mA (corresponding to the lower limit of the human “let-go” threshold of current), and which will trip within several seconds of a ground-fault in excess of a current level of 6 mA, or within 0.025 to 0.100 second for ground-fault current in excess of 20 mA to 30 mA.
Furthermore, there is a need for a GFCI system that will quickly determine which line has been faulted and will interrupt the feeder circuit including that line without interfering with the operation of other feeder circuits in the system.
In addition to the advantages of the GFCI system described above to avoid fatal shocks, incipient failure of electrical insulation can also be detected at a current sensitivity of 6 to 30 mA, which can minimize equipment damage.